In recent years, fuel efficiency has been strongly sought from automobile tires due to environmental issues and resource issues. On the other hand, excellent wet grip is being sought from the viewpoint of safety. A cross-linked product of a rubber composition including silica as a filler is better in low heat buildup compared with a cross-linked rubber of a rubber composition including carbon black, so the rolling resistance in the case of forming a tire becomes lower. For this reason, by using a cross-linked rubber of a rubber composition containing silica to form a tire, it is possible to obtain a tire excellent in fuel efficiency.
However, even if mixing silica into conventional rubber, rubber and silica are insufficient in compatibility, so easily separate. Due to this, the rubber composition before cross-linking is poor in processability and, further, the cross-linked rubber obtained by cross-linking this becomes insufficient in low heat buildup.
Therefore, to improve the compatibility between rubber and silica, for example, it has been proposed to add the various silane coupling agents such as disclosed in Patent Document 1 or Patent Document 2 etc. into a rubber composition. However, handling a silane coupling agent requires sophisticated processing technique. Further, a silane coupling agent is expensive, so if the amount becomes greater, there is the problem that the manufacturing cost of the tire will become higher.
To solve such a problem, for example, as disclosed in Patent Documents 3 to 5 etc., when using the solution polymerization method to obtain a polymer of rubber, the technique of causing a modifier to react with an active end of the polymer to thereby impart compatibility with silica to the rubber itself has been studied. However, due to the rising demands for fuel efficiency and wet grip from automobile tires in recent year, rubber able to give cross-linked rubber further better in low heat buildup and wet grip has been desired.